The present invention relates to high-frequency cables, and more particularly to a multi-layer high-frequency or coaxial cable and a method for manufacturing the same.
Coaxial cables and other high frequency cables are known in the art for transmitting, for example, television signals and other communication signals. As shown in FIG. 1, a conventional coaxial cable 100 is formed out of an inner tube 102, a dielectric material 103, and an outer tube 104. The two tubes 102, 104 are made of metal or another electrically conductive material and are disposed concentrically with the dielectric material 103 sandwiched in between the two tubes. The conductivity of the material used to form the tubes 102, 104 and the relative permittivity and dissipation factor of the dielectric material 103 will determine the RF attenuation of the resulting coaxial cable. As is known in the art, at radio frequencies the current flowing through the tubes 102, 104 in the cable 100 tends to flow only in and directly beneath the surfaces of the conducting tubes. This is commonly known as a xe2x80x9cskin effectxe2x80x9d. More particularly, current flows through and directly beneath an inside surface of the outer tube 102 and an outside surface of the inner tube 104.
Each tube 102, 104 is manufactured by bending a flat strip of conductive tape into a round tube and welding the longitudinal edges of the tape together to form a seam. To minimize manufacturing costs, the material selected for forming the tubes 102, 104 is preferably one that is easy to form and weld. However, the materials that provide the best manufacturing characteristics do not necessarily offer the conductivity required for minimizing RF attenuation.
There is a need for a coaxial cable that has high conductivity, to minimize RF attenuation, and yet preserves the ease of manufacture and welding provided by less conductive materials.
Accordingly, the present invention is directed to a coaxial cable apparatus and method having a dual-layer structure for both its inner and outer tubes. Each tube is formed out of a flat strip having a base layer and a high conductivity layer disposed on the base layer. In a preferred embodiment, the high conductivity layer is disposed on less than the entire surface of the base layer, leaving the margins on the longitudinal edges of the base layer free of high conductivity material to form edge clearances. The flat strip is then bent to form a tube, with the edges of the tube being welded together. The edge clearances allow the edges of the base layer to be welded together without the weld joint touching the high conductivity layer, thereby avoiding potential problems associated with welding materials having different metallurgical properties. The preferred coaxial cable structure is arranged so that the high conductivity layer is on the outer surface of the inner tube and on the inner surface of the outer tube, following the normal current flow pattern in coaxial cables. By providing a separate high conductivity layer for each tube, the inventive dual-layer structure improves the RF attenuation characteristics of the coaxial cable while preserving the ease of manufacture provided by the material used in the base layer.